A CME should be impossible there - nothing in EnchantedFurnace is done off the main thread aside from saving, so there's no way the collection should be modified during iteration. I'd try re-downloading the plugin and restarting the server to see if that fixes it. I cannot replicate this running git-Spigot-d21162c-61e0c69, will update tonight and test on git-Spigot-d21162c-82f3b02.

Edit: Unable to reproduce on git-Spigot-d21162c-82f3b02.

Re: startup error - I guess I'll push out a compatibility build soon-ish. I haven't had the time to finish the huge save system overhaul unfortunately, though I've finally managed to pick up some projects again.

I'm aware, I never pushed a fix for the startup warning because the runnable version works well enough and I was in the middle of rewriting a good chunk of the plugin. I'm worried about the CME, not that.

Runnables work fine, don't worry. NMS furnace support has actually been broken since midway through 1.12.1. I've been ever so slowly putting in time on a major rework, so I haven't spent the time to update the version-specific handling.